Florian,
Check out this topic:viewtopic.php?f=196&t=2384026
it might help you a bit.
I would recommend to keep the back strap in the centerline of the board and front straps' inner inserts 8-9cm apart. 6 might be not enough.

I would make it 5cm wider.
50cm might be not enough in winds below 8kts (sideshore) or below 10kts (onshore) especially for guys over 85kg.
Some raceboards are 60cm wide. One of my prototypes was so wide but it was too wide for 93cm long foil. However 60cm might be good for foils 100cm+.

Thanks everyone for your valuable comments, they are much appreciated!

Here is another sketch with the collected informations for anybody to fall back on when designing your Race board.

@Bradn: Looking good, I think 55cm is the way to go. This version has 43L too.

Regarding the bottom edges I was going for sharp edges on my first board, thinking I need them to "grip" for extra upwind abillity. After I spend the last 10 sessions learning to foil I quickly realised this is not the case, in fact it's producting spray and catches the edge occasionally.

The chamfered edges will prevend this and I believe they are more forgiving on high speed touch downs. An a last note I think the edge grip on a foilboard is not that important as there is not much leeward drift (mast) and it's pivoting point is the tail (mast); correct me if I'm wrong!!!